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The principle behind the usefulness of cationic liposomes in cellular transfection was studied using fluorescence microscopy and freeze-fracture microscopy. The experimental model used included large unilamellar vesicles (LUVs) composed of 1,2-dioleoyl-sn-phosphatidylethanolamine and N,N-dimethyl-N,N-di-9-cis-octadecenylammonium chloride which were made to fuse with target liposomes. Membrane fusion was found to be enhanced by increased negative surface charge while that for liquid crystalline lipids, by increased acyl chain unsaturation in target liposomes.

31P-nuclear magnetic resonance and freeze-fracture electron microscopy were used to determined the effects of poly(ethylene glycol)(PEG)-lipid conjugates on the polymorphic behavior of dioleoylphosphatidylethanolamine and cholesterol mixtures. The mixtures adopted a hexagonal phase when hydrated, but changed to a bilayer conformation when PEG-lipid conjugates were added to the mixtures. The conjugates increased the temperature of the mixtures during their transition from the hexagonal to the bilayer phase. They also increased the proportion of phospholipids in the bilayer phase.

The influence of poly(ethylene)glycol (PEG)-lipid conjugates on calcium-induced liposomal fusion into large unilamellar vesicles was investigated using a combination of vesicle fusion and fluorescence resonance energy transfer assays. The results revealed a direct relationship between degree of inhibition of liposomal fusion and the molar ratio of the PEG conjugate and size of the PEG moiety. Inhibition was due to the strong opposition in bilayer and interbilayer surface membrane contact. However, fusogenic activity was restored upon the addition of neutral acceptor liposomes.

A study was conducted to investigate in vivo membrane-protein interactions in blood using a novel spin column procedure. The results revealed strong association of blood proteins with liposomes composed of long-chain saturated phosphatidylcholines. This protein-liposome interaction was rapidly cleared from the circulation. It was regulated by the presence of cholesterol, which reduced the number of proteins binding to the unilamellar vesicles.

Freeze-fracture electron microscopy and fluorescent lipid-mixing assay experiments reveal the fusion of lipid bilayers that consist of synthetic amino lipids and that the introduction of transbilayer asymmetry into the amino lipids through modified pH gradients helps modulate the fusion process. Liposomes with 5 mol % 1, 2-dioleoyl-3-(N,N-dimethylamino)propane (ALI), dioleoylphosphatidylethanolamine and egg phosphatidylcholine mixtures do not display fusion at 4.0 pH, but do so on pH degradation and ALI rearrangement. The presence of longer unsaturated acyl chains promotes the fusion rate of the lipid layers.

Basic peptides with identical amino acid compositions and varying amino acid sequences show varied lipid bilayer permeabilities influenced by charge distribution and the degree of hydrophobicity of the transbilayer movement of these molecules. Peptides with segregated fundamental functions at one end of the molecule have improved capacity for translocation across biological membranes.

The role of lipid asymmetry in the regulation of Ca2+-induced membrane fusion was investigated. It was shown that lipid asymmetry can regulate fusion phenomena between large unilamellar vesicles, composed of dioleoylphosphatidylcholine, dioleolyphosphatidylethanolamine, phosphatidylinositol and dioleoylphosphatidic acid, and that the composition of the outer monolayer determines the rate and extent of fusion.

A study was conducted on the influence of transmembrane pH gradients on the Ca2+-induced fusion of phospholipid vesicles possessing free fatty acids. A kinetic assay for lipid mixing was used to measure fusion. Results showed that the vesicles displayed fusion properties similar to those without oleic acid when a pH gradient was present. Observations of the kinetics of the fusion process also reveal the rapid inward movement of oleic acid in response to the pH gradient.